This invention relates to a method of grinding a side end surface of a substrate, such as a glass substrate for a magnetic recording medium, and, in particular, to the method suitable for grinding an inner peripheral side end surface and an outer peripheral side end surface of the glass substrate. The inner and the outer peripheral side end surfaces might be simply called inner and outer side end surfaces, respectively, hereinafter. In addition, it should be noted throughout the instant specification that the terms xe2x80x9cgrindxe2x80x9d, xe2x80x9cgrindingxe2x80x9d and xe2x80x9cgroundxe2x80x9d will be used to process the side end surface and to distinguish the end surface processing from the any other processing, especially, main surface processing, but they should be practically interpreted as the meaning of xe2x80x9cpolishxe2x80x9d, xe2x80x9cpolishingxe2x80x9d, and xe2x80x9cpolishedxe2x80x9d, respectively.
In general, an aluminum substrate has been widely used as a magnetic recording medium substrate for a magnetic disk or the like. However, it has been a recent trend that such an aluminum substrate has been gradually replaced by a glass substrate which is excellent in flatness and strength in comparison with the aluminum substrate, as the magnetic disk becomes smaller in size, thinner in thickness, and higher in recording density.
As such a glass substrate for the magnetic recording medium, use has been made of a chemically reinforced glass substrate and a crystallized or devitrified glass substrate reinforced by recrystallization or devitrification.
As the recording density becomes higher, a magnetic head has also been changed from a thin film head to a MR (magneto-resistive) head or a GMR (giant magneto-resistive) head. From this fact, it will be expected that the current of times will be directed to reproducing on the magnetic recording medium of the glass substrate by using the magneto-resistive (MR) head.
Under the circumstances, a wide variety of improvements have been made in the magnetic disk so as to realize a high recording density. With such improvements of the magnetic disk, various new problems have taken place one after another in connection with the glass substrate for the magnetic recording medium (will be simply called a glass substrate hereinafter), also. One of the problems is that a substrate surface of the glass substrate must be kept thoroughly clean. For example, when foreign materials are adhered to the substrate surface, they bring about film defects of films deposited on the substrate surface and/or cause projections to occur on the films. In addition, the projections on the films make it difficult to maintain an appropriate glide height.
On the other hand, when the magneto-resistive (MR) head is used for reproducing from the magnetic recording medium of the glass substrate a flying height of the MR head tends to be lowered to improve a recording density. In this event, it often happens that phenomena take place such that reproduction is disordered or becomes impossible. It has been pointed out that the cause or source of the phenomena reside in projections formed by particles adhered to the substrate surface of the glass substrate. Specifically, the projections on the magnetic recording medium give rise to thermal asperity and bring about a temperature rise on the MR head. Such a temperature rise on the MR head results in varying a resistance value of the MR head and adversely affects electro-magnetic conversion.
Now, it is well known that the glass substrate for the magnetic recording medium is shaped like in a ring and, therefore, has an inner side end surface and an outer side end surface which may be referred to as inner and outer side peripheral surfaces, respectively. In addition, the magnetic recording medium is usually housed in a resin case.
As the source of the foreign materials on the glass substrate, recent attention has been directed to particles that occur due to undulations of the side end surfaces of the glass substrate. Specifically, when the magnetic recording medium with the glass substrate is rotated in the resin case, the outer side end surface of the glass substrate rubs a wall of the resin case on account of the undulations of the outer side end surface of the glass substrate. Such rubbing brings about adhesion of the particles or resin, glass, and the like to the magnetic recording medium. Moreover, any other particles are often adhered to the magnetic recording medium from the inner and the outer side end surfaces of the glass substrate.
Especially, it has been found out by the instant inventors that the inner side end surface of the glass substrate is rough or equivalent in flatness as compared with the outer side end surface and is therefore susceptible to cause undesired particles.
As regards a chemically reinforced glass substrate as mentioned before, chemical reinforcement is carried out by immersing a glass substrate into a chemical reinforcement or strengthening solution that is heated. On immersion, the glass substrate itself is held in a holder (for example, three points holder). Specifically, a plurality of the glass substrates are supported by the holder at support portions of the outer end side surface of the glass substrates. Herein, it is assumed that the outer end side surfaces of the glass substrates are not flat especially at the support portions. In this case, undesired liquid sumps of the chemical reinforcement solution are liable to occur at the support portions when the glass substrates are lifted from the chemical reinforcement solution .Such liquid sumps leave, on the support portions, particles resulting from foreign materials included in the chemical reinforcement solution and finally result in occurrence of the thermal asperity due to the particles.
Conventionally, a method and an apparatus for grinding inner and outer side end surfaces of a glass substrate are disclosed in Japanese Patent Unexamined Publication Nos. H11-33886 (33886/1999) and H11-28649 (28649/1999). In each conventional grinding method, a plurality of glass substrates or glass disks are stacked on a rotatable table and are rotated in a predetermined direction. Grinding is performed by supplying the rotating glass substrates with slurry including an abrasive material and by rotating a grinding brush in a direction reverse relative to the predetermined direction of the glass substrates. Furthermore, the grinding brush is reciprocated in a stacked direction of the glass substrates. Thus, the inner and outer side end surfaces can be ground.
Such grinding the side end surfaces is generally performed with the glass substrates directly stacked together before the glass substrates are chemically reinforced. This shows that un-reinforced or pre-reinforced glass substrates are ground by the grinding brush with a pressure imposed onto the side end surfaces of the glass substrates by the grinding brush. As a result, the pressure of the grinding brush is directly given to the glass substrates stacked one another and often brings about breakage of the glass substrates. In order to avoid the breakage of the glass substrates, the grinding of the side end surfaces (may be called side surface grinding) must be performed when each glass substrate is thick to some extent. Taking this into consideration, the side surface grinding is usually carried out before a final lapping process for lapping or polishing each glass substrate by the use of a carrier including abrasive grains.
Herein, let the final lapping process be carried out after the side surface grinding. In this event, each glass substrate is lapped or rubbed during the final lapping process not only on a main surface thereof but also on the side end surface. This is because the abrasive gains undesirably lap or rub the side surfaces of each glass substrate due to an interference between the abrasive material used in the side surface grinding and the abrasive grains used in the final lapping process.
Since the side end surfaces are once mirror finished by the side surface grinding, the side end surfaces are roughened again by the abrasive grains during the final lapping process and are not polished until completion of an end product. This shows that the end product has the roughened side end surfaces. As a result, the thermal asperity inevitably takes place in the end product manufactured in the manner mentioned above. In addition, such an end product is disadvantageous in that it is weak in transverse rupture strength.
As readily understood from the above, the glass substrates are directly stacked during the side surface grinding without any spacing between two adjacent ones of the glass substrates. However, an unground portion inevitably remains on the side end surfaces of each glass substrate when the above-mentioned method is used. Such an unground portion gives rise to the thermal asperity and weakens the transverse rupture strength.
It is an object of this invention to provide a method of effectively grinding side end surfaces of each glass substrate without any breakage even when a thickness of each glass substrate is thin.
It is another object of this invention to provide a method of the type described, which can perform a side surface grinding process after a final lapping process.
It is still another object of this invention to provide a method of the type described, wherein an unground portion is not left on the side end surfaces of each glass substrate and subsequently the side end surfaces can be kept thoroughly clean at a low cost.
It is yet another object of this invention to provide a glass substrate for a magnetic recording medium, which has a main surface kept thoroughly clean. It is another object of this invention to provide a recording medium which can ultimately avoid troubles resulting from foreign materials adhered to the main surface.
According to a first aspect of this invention, a method is for use in grinding each side end surface of a plurality of substrates of disk shapes. The method comprises the steps of successively stacking the substrates with an intermediary interposed between two adjacent ones of the substrates and grinding each side surface with the intermediary interposed between the substrates.
With this method, the substrates are spaced apart from one another in a main surface direction with the intermediary interposed between adjacent ones of the substrates. No pressure is imposed on both side end surfaces of the substrates doe to a grinding brush or pad on the side end surface grinding. This shows that no breakage takes place on the side end surfaces of the substrates during the side end surface grinding even when each substrate is thin. Accordingly, each substrate can be ground even when each substrate is kept in a thin state (for example, 0.5 to 1.3 mm) after it is subjected to a lapping process. Even when each substrate can be polished after the side end surface grinding, the side end surface is not roughened because abrasive grains in the polishing process are smaller than those in the side end surface grinding. In addition, the substrates spaced via the intermediary have chamfered portions ground without any remnant portions left unground because such chamfered portions are also ground by the grinding brush or so. Thus, it is possible with this method that the side end surface can be kept in a highly smoothed state at a low cost. This makes is possible to keep such main surface of the substrates highly clean and to strengthen the transverse rupture strength.
During the side end surface grinding, free abrasive grains must be brought into contact with the side end surface. Especially, when the substrate is a glass substrate of a disk shape used for a magnetic recording medium, both inner and outer side end surfaces must contact as the with the abrasive grains. In order to bring both the inner and the outer side end surfaces into contact with the abrasive grains, use may be made of a method of immersing a grinding solution including the abrasive grains or method of directly supplying the grinding solution onto the inner and the outer side end surfaces by spraying the grinding solution or the like. Either one of the above-mentioned methods may be used. The former method may not be considered in connection with a lack of the grinding solution.
At any rate, the above-mentioned grinding method realizes the glass substrate of a surface roughness that can avoid thermal asperity, namely, a surface roughness less than 0.2 xcexcm and 1.0 xcexcm when it is represented by Ra and Rmax, respectively, respectively.
According to a second aspect of this invention, a method is for use in processing a plurality of substrates of disk shapes, each of which has a pair of main surfaces and inner and outer side end surfaces contiguous to the main surfaces. The processing method comprises the steps of lapping each main surface of the substrates, polishing each main surface of the substrates, successively stacking the substrates, and grinding at least one of the inner and the outer side end surfaces with the intermediary interposed between the substrates. The side end surfaces of the substrates ground by the side end surface grinding are kept clean during the following process, such as the polishing process. Thus, the method is very effective to keep the side end surfaces and the main surfaces in a highly clean or smooth state.
According to a third aspect of this invention, the substrates are spaced apart from each other through the intermediary which may be a spacer. Such intervention of the spacer serves to certainly prevent breakage of the substrates during the side end surface grinding and to avoid occurrence of remnant portions unground by a grinding brush on chamfered portions of the inner and the outer side end surfaces. The spacer may not be restricted by size and configuration when the adjacent substrates are kept in parallel to each other by the spacer. For example, the spacer may have a ring shape, a bay shape partially cut away from the ring shape, or may be divided into a plurality of small circular spacers without any hole. The size of the spacer may be equal to or slightly smaller than that of each substrate and may not exceed the chamfered portion or portions of each substrate. The thickness of the spacer may be adjusted in accordance with diameters of bristles of the grinding brush and preferably falls within a range between 0.1 and 0.3 mm. The grinding method according to this invention is available for grinding an outer side end surface of a substrate which has no center hole. In this case, it is needless to say that the spacer may have no center hole.
According to a fourth aspect of this invention, the spacer may be formed by a soft material such that breakage of each substrate is not caused to occur due to a pressure resulting from the grinding brush or the grinding pad. Using such a soft material as the spacer makes it possible to grind the side end surfaces of the glass substrates even when each substrate is thin in thickness. This means that the grinding step can be performed after the lapping step and before the polishing step. As a result, the side end surfaces of the substrates ground are not roughened by the polishing step and are kept clean.
For a portable telephone set, a digital camera, a navigation device, and the like, proposals recently have made about a small substrate of 1 inch in diameter less than 2.5 inches. Since such a small substrate is very thin in thickness, it is very important to grind the side end surfaces without breakage and to keep the surface state clean after the side end surfaces.
Although a material of the spacer is not restricted, it is preferable that the spacer may be formed, for example, by polyurethane, acrylic acid resins, plastics, rubber, or the like. For instance, when the spacer is formed by the same material as the grinding pad, it may have the Shoe A hardness between 15xc2x0 and 90xc2x0.
As mentioned before, the side end surface grinding step is preferably performed after the lapping step so as to keep the surface states of each side end surface clean. Specifically, the lapping step is performed by using the abrasive grains more than 0.5 xcexcm and preferably between 0.5 xcexcm and 2 xcexcm while the grinding step is performed by the use of abrasive grains between 1 xcexcm and 4 xcexcm so as to avoid the thermal asperity.
According to a sixth aspect of this invention, the grinding step comprises the steps of grinding the inner side end surface of each substrate; and grinding the outer side end surface of each substrate. In this event, either one of the inner and the outer side end surfaces is performed prior to the other. Alternatively, the inner and the outer side end surfaces may be performed simultaneously.
It is preferable that the inner side end surface is ground and thereafter the outer side end surface is ground. This is because the step and an apparatus for grinding the outer side end surface can be utilized on grinding the inner side surface also. More specifically, when the outer side end surface is at first ground prior to the inner side end surface, provision is to be made about both a grinding jig for grinding the outer side end surface and another grinding jig for grinding the inner side end surface. On the other hand, when the inner side end surface is at first ground, a grinding jig for grinding the inner side end surface can also be used as a grinding jig for grinding the outer side end surface.
Furthermore, when both the inner and the outer side end surfaces are simultaneously ground, jigs and apparatus should be improved. However, this method is advantageous in that a cost can be reduced when a comparatively small number of the substrates are ground.
When the inner side end surface is ground after the outer side end surface is ground, a contact force or pressure imposed on the glass substrate from the grinding brush or pad becomes small on the inner end surface grinding as compared with the outer side end surface grinding. Therefore, this method is effective to avoid breakage of the substrates on the inner side end surface grinding and to prevent a reduction of a yield due to the breakage.
According to a seventh aspect of this invention, the inner side end surface grinding step is performed at a pressure which is imposed by a grinding brush or a grinding pad onto the inner side end surface of each substrate and which is less than a pressure imposed during the outer side end surface grinding step. Such a reduction of the pressure imposed on the inner side end surface grinding step can avoid fluctuation of an axis of the grinding brush or pad used for the inner side end surface grinding step. Accordingly, the inner side end surface can be machined at a high precision in configuration and state. Specifically, the pressure of the grinding brush or pad imposed onto the glass substrates falls within a range between 0.05 and 0.3 MPa in the inner side end surface step while it falls within a range between 0.05 and 0.5 MPa in the outer side end surface.
According to an eighth aspect of this invention, the inner side end surface grinding step is performed by a grinding brush which has bristles having diameters smaller than those of bristles of a grinding brush used in the outer side end surface grinding step. Using such grinding brushes is effective to further improve the surface state. The diameters of the bristles of the brushes may be selected between 0.05 mm and 1.0 mm in consideration of a length of the chamfered portion. Specifically, when the glass substrate has a thickness of 0.9 mm and the length of the chamfered portion of 0.28 mm, the diameters of the bristles of the grinding brush for grinding the outer side end surface fall within a range between 0.08 and 0.3 mm (preferably, 0.2 and 0.3 mm) while the diameters of the bristles of the grinding brush for the inner side end surface fall within a range between 0.08 and 0.3 mm (preferably, 0.1 and 0.3 mm).
On the other hand, when the grinding pad is used, a hardness of the pad is adjusted. Specifically, the grinding pad for the outer side end surface may have a Shoe A hardness between 50xc2x0 and 90xc2x0 while the grinding pad for the inner side end surface may have the Shoe A hardness between 25xc2x0 and 90xc2x0.
It is needless to say that the diameters of the bristles for the inner and the outer side end surfaces may be optionally determined in view of another aspect different from the eighth aspect.
According to a ninth aspect of this invention, a method is for use in manufacturing a glass substrate for a magnetic recording medium and comprises the step of grinding the inner and/or the outer side end surfaces. The glass substrate has a surface kept highly clean and an excellent transverse rupture strength.
According to the tenth aspect of this invention, the magnetic recording medium is structured by the glass substrate which is excellent in cleanliness and transverse rupture strength. In the magnetic recording medium, it is possible to reduce defects on each layer deposited on the glass substrate and to lower a glide height because the glass substrate has no foreign materials attached onto the side end surfaces thereof.
According to an eleventh aspect of this invention, the magnetic recording medium is used for an MR head or a GMR head and can avoid malfunction resulting from the thermal asperity and wrong reproduction.
According to a twelfth aspect of this invention, the magnetic recording medium has a magnetic layer comprising Co and Pt. The magnetic recording medium mentioned before has excellent magnetic characteristics.
According to a thirteenth aspect of this invention, a glass substrate is for use in a magnetic recording medium and has a pair of main surfaces and inner and outer side end surfaces each of which is contiguous to the main surfaces and which have inner and outer side end surface portions. The outer side end portion is ground together with the inner side end portion and has a surface roughness smaller than the inner side end portion. Each of the inner and the outer side end portions is contiguous to the main surfaces through a chamfered portion between the side end surface portion and each main surface. Each chamfered portion is also ground. According to a fourteenth aspect of this invention, each of the inner and the outer side end portions has a surface roughness Rmax not greater than 1 xcexcm. With this structure, the glass substrate is subjected to side end surface grinding and is thereafter chemically reinforced. Such a chemical reinforced glass substrate can avoid occurrence of liquid sumps of chemical reinforcement solution at support portions to be supported by a holder. Therefore, it is possible to prevent occurrence of the thermal asperity due to foreign particles included in the chemical reinforcement solution.